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Blood and bone features in JAK2V617F mutated prefibrotic essential thrombocythemia (ET) and polycythemia vera (PV) are overlapping in terms of increased cellularity due to increased erythropoiesis and pleomorphic megakaryocytes indicating the need to measure red cell mass (RCM) according to PVSG and WHO criteria. The JAK2V617F mutated myeloproliferative neoplasms (MPN) appeared to be a broad biological continuum of normocellular ET, ET with features of polycythemia vera (prodromal PV), classical PV, advanced PV, Inapparent PV (IPV) with splenomegly, masked PV or hypercellular ET due to megakaryocytic granulocytic myeloproliferation (ET.MGM) when the 2013 WHO and European Clinical, Molecular and Pathological (WHO-CMP) criteria are applied. The megakaryocytes morphology may change from pleomorphic to dysmorphic in advanced PV, in IPV and in masked PV (ET.MGM) as bone marrow cellularity, the degree myelofibrosis and the JAK2V617F mutation load increase during long-term follow-up. Erythrocytes above the upper limit of normal (5.8×1012/L), but not hemoglobin and hematocrit is clearly correlated with increased red cell mass (RCM) above (30) ml/kg) and therefore diagnostic for PV and idiopathic erythrocythemia (IE) on top of pathognomonic MPN bone marrow histology in newly diagnosed PV patients with normal mean cell volume (MCV) of erythrocytes and no or minor of splenomegaly. Erythrocyte count remain above the upper limit of normal in PV and IE in complete hematological remission by phlebotomy alone as the consequence of iron deficiency induced microcytic erythrocytes that correct the blood volume (RCM), hemoglobin and hematocrit to normal. The combination of increased RCM, increased plasma volume, and normal or low erythrocyte counts is characteristic for IPV with significant splenomegaly as the only cause of increased blood volume without symptoms of hypervolumemia.